The method of the invention is intended for use particularly in PMR (Private Mobile Radio) networks, or trunking networks, which are typically company networks or public safety and security networks, in which all the channels are allocated to one or more companies or authority organizations. In addition to subscriber numbers, the subscribers in these networks have been assigned group numbers indicating to which group call group or subscriber group the subscriber belongs; thus it is possible to switch calls directed to the members of a certain group to all the subscribers of this group.
A group call is one of the key functions in a PMR network. A group call is used in various activities in which several people participate, particularly when a whole group must continuously be aware of the progress of events. A group call is a call, in which all participants can in turn talk and listen to each other. In group calls the whole group is called by one radio identity code. It is known that many radio telephone systems, particularly systems used by companies and authorities, apply group calls. As regards the radio path, a group call is typically implemented in simplex form as a point-to-multipoint call, in which speech is transmitted from one talking party to many talking parties, and a speech item is allocated to the following talking party according to a predetermined practice.
A call or a group call refers to complete exchange of information between two or more parties. A call may consist of one or more speech items. In a semi-duplex call, these speech items are sequential. A speech item refers generally to all functions relating to complete unidirectional transmission of information during a call.
The invention is applicable in mobile communication systems with either digital or analog radio paths. Analog mobile communication systems are disclosed, for example, in MPT 1327, A Signalling Standard for Trunked Private Land Mobile Radio Systems, January 1988, revised and reprinted November 1991, and MPT 1343, Performance Specification, January 1988, revised and reprinted September 1991, both issued by the Radio-communications Agency, published by the British Department of Trade and Industry.
An example of digital radio telephone or mobile communication systems is the TETRA system (TETRA=Trans-European Trunked Radio). Its implementation is disclosed in the standard prETS 300 392-2, November 1995, Radio Equipment and Systems (RES); (TETRA); Voice plus Data (V+D) Part 2: Air interface, ETSI, 579 pages. In this system, the allocation of speech items is controlled by a switching and management infrastructure (SwMI) through base stations connected to it. When the operation of a switching and management infrastructure and base stations is referred to, only the term `base station` will usually be employed hereafter for reasons of simplicity. The allocation of speech items in point-to-multipoint operation is thus controlled by a base station, which typically grants speech items on the basis of the requests made by mobile stations (MS). In this arrangement, a mobile station cannot transmit speech without a permission given by the base station.
Particularly in security services, a user of a mobile station may get into an emergency or some other situation requiring urgent communication or priority connections. To this end, the user of a mobile station may, when requesting a speech item, define the importance or urgency of the speech item in a speech item demand message, for instance by means of a four-step scale. The highest two values of the scale are defined as pre-emptive, i.e. the base station may force a mobile station transmitting at that moment to stop the transmission and allocate a speech item to the mobile station that requested a more urgent speech item.
The TETRA standard further defines talking party identification (SS-TPI), by means of which the identifier of the talking party, e.g. the subscriber number or a short character sequence, is shown to the other participants in a group call in order to identify the talking party or the talking party's function. This supplementary service is described in the standard prETS 300 392-10-3, September 1994, Radio Equipment and Systems (RES); (TETRA); Voice plus Data (V+D) Part 10: Supplementary services stage 1 Part 10-3 Talking Party Identification, ETSI, 16 pages. The corresponding operation may be implemented as only a part of the complete talking party identification supplementary service: the basic operation of the service, i.e. indicating the identifier, can be used without a subscription agreement or without identifier contents defined directly by the user.
Furthermore, the TETRA standard prETS 300 392-2, November 1995, Radio Equipment and Systems (RES); (TETRA); Voice plus Data (V+D) Part 2: Air interface, ETSI, particularly item 14.7.2.7, U-STATUS, defines a status message service, by means of which a mobile station can send one of predetermined status messages to the other participants in a group call, for example. At present, the standard defines one type of status information, "emergency", in addition to which the user or the network operator may define other types of status information.
In the case of an emergency group call, it may be difficult for other subscribers to distinguish between the speech items of subscribers in an emergency and those of the other subscribers. The subscriber who started the call cannot always be unambiguously considered to be in an emergency; it is also possible that several participants in the call are in an emergency or that the call has been started by another subscriber, whereby the emergency call begins only when the subscriber/subscriber station in the emergency requests an emergency speech item.